1. Field of the Invention
The present invention relates to the reduction of the rate of radiation exposure to radiation sensitive elements located outside of a nuclear reactor core. In particular applications the invention relates to the reduction in the rate of radiation exposure to welds in a reactor pressure vessel.
2. Description of the Prior Art
In certain nuclear reactor applications it has proven desirable to reduce the rate of radiation exposure to particular locations external of the reactor core; i.e., "excore locations". An example of this situation occurs in the welds of nuclear reactor pressure vessels that are normally subjected to relatively high levels of neutron flux. It has been found that radiation exposure of the welds over a period time may cause the weld material to become brittle to the extent that the exposure must be reduced in order to allow the reactor to operate to the end of its design life.
Previous designs of fuel assemblies that provide acceptable reductions in the radiation exposure rate result in inordinate penalties with regard to thermal limits, cycle length, operating flexibility and/or power level of the reactor. Prior proposals have included the use of so-called "dummy" assemblies which contain no fuel and the complete removal of fuel assemblies at critical locations.
Other more drastic and costly remedies include (i) annealing the pressure vessel walls after reactor shut down and the removal of all fuel elements and (ii) heating the supply of emergency cooling water in order to reduce thermal shock to the weld should the cooling water be circulated.
Accordingly, there is an acute need for a design that will allow a maximum reduction in the radiation exposure rate in the desired excore locations while minimizing the above-recited penalties in performance and cost.